**6.1 Effect of chitosan on UASB treating POME**

It has been reported that thermophilic operation of anaerobic reactors provides some advantages over mesophilic operation in areas such as higher rates of substrate degradation and biogas production. However, mesophilic reactors can be preferable because of greater process stability (Mustapha et al., 2003; Poh & Chong, 2009). Operating temperature is a major factor that greatly influences digester performance (Choorit & Wisarnwan, 2007; Poh & Chong, 2009; Yu et al., 2002).

The effects of chitosan as a sludge granulation accelerator during the transition from mesophilic (37oC) to thermophilic condition (57oC) has been investigated by Khemkhao et al. (2011). They used two UASB reactors, with a working volume 5.3 L, both of which they inoculated with mesophilic anaerobic sludge. The sludge was then acclimatized to a thermophilic condition with a stepwise temperature increase of 5oC from 37 to 57oC. The OLR ranged from approximately 2 to 9.5 g COD/L·d. One of the reactors was then injected with a chitosan dosage of 2 mg chitosan g/VSS on the first day of operation and the second reactor was used as a control.

At all times during the operation of the two reactors, the UASB with chitosan addition was found to have 5% higher COD removal efficiency and 16 L/d higher biogas production rate (7.82 L/g VSS removed·d) than that of the control. The methane contents of both reactors were found to be similar, with approximately 78% methane content for UASB with chitosan addition and 76% for the control. The effluent VSS in both reactors was found to increase with increase of OLR. The UASB with chitosan addition was found to have 6 to 23% lower effluent VSS than that of the control. Khemkhao et al. (2011) concluded that the UASB with chitosan addition had consistently better performance than the control.

Enhancing Biogas Production and UASB Start-Up by Chitosan Addition 337

Hydrogenotrophic methanogens convert H2 and CO2 into CH4. Acetotrophic methanogens convert acetate into CH4 and CO2. The acetotrophic methanogens grow slower than the acid-forming bacteria. About two-thirds of CH4 is derived from acetate conversion by acetotrophic methanogens. The other third is the result of H2 and CO2 reduction by

As stated above (Khemkhao et al., 2011), lower biomass washout was observed from the UASB with chitosan addition than from the control, especially at higher biogas production rates. The DGGE analysis shows that UASB with chitosan addition contains higher populations of *Methanosaeta* species than the control. It can be concluded that the chitosan helped to retain these methanogens, thus resulting in higher populations of acetotrophic

Tiwari et al. (2005) and Tiwari et al. (2006) have reported that acetotrophic methanogens significantly accelerate granule development. Higher population of acetotrophic methanogens

Chitosan has been reported to act like an ECP in enhancing the aggregation of acidogens. As shown in Fig. 7, the aggregated acidogens then form granules with highly elastic outer

Fig. 7. Scheme of granule formation. Top: Surface tension model according to Thaveesri et al. (1995) and Hulshoff Pol et al. (2004). Middle: Some circumstances in the control reactor.

Bottom: Enhanced aggregation by chitosan in UASB with chitosan addition (from

may in turn lead to higher methane production in the reactors with chitosan addition.

hydrogenotrophic methanogens.

methanogens.

Khemkhao et al., 2011)
